Non-cryogenic bulk oxygen separation systems, for example, organic polymer membrane systems, have been used to separate selected gases from air and other gas mixtures. Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. An entirely different type of membrane, however, can be made from certain inorganic oxides. These solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure.
Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500.degree. C., generally in the 600.degree. C. to 900.degree. C. range. This limitation remains despite much research to find materials that work well at lower temperatures. Solid electrolyte ionic conductor technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
It is relatively easy to use the basic ion transport separation process to remove nearly all of the oxygen from the feed gas stream to produce a nitrogen product gas stream, particularly if the permeate side of the ion transport membrane can be purged with an oxygen-free stream. It is, however, more difficult to efficiently recover oxygen as the product using this basic process. For example, if pure oxygen is withdrawn from the permeate gas stream at atmospheric pressure, the feed gas stream must be at a pressure well in excess of 5 atmospheres in order to continue to drive oxygen through the ion transport membrane. Thus, most of the compression energy is lost in the retentate gas stream or nitrogen waste stream unless that gas stream represents a product which is required at pressure.
Another alternate process involves vacuum pumping of the permeate side of the ion transport membrane in order to maintain the driving force for the permeation process without contaminating the oxygen product gas stream. However, there is a considerable cost to operating the vacuum pumps.
Advances in the state of the art of air separation using solid electrolyte ionic conductors have been presented in the technical literature. U.S. Pat. No. 5,306,411 (Mazanec et al.) discusses mixing an inert diluent such as steam with a light hydrocarbon feed gas to produce synthesis gas or unsaturated hydrocarbons as a permeate effluent from the anode side of an ion transport membrane. An oxygen-containing gas is passed through a retentate or cathode side of the membrane; the resulting oxygen-depleted gas withdrawn from the retentate zone apparently is discarded. Mazanec at al. further disclose in U.S. Pat. No. 5,160,713 that steam can be generated in the permeate zone by reaction between hydrogen, introduced as an oxygen-consuming substrate, and oxygen transported through the membrane.
U.S. Pat. No. 5,565,017 (Kang et al.) relates to a system integrating an ion transport membrane with a gas turbine to recover energy from the retentate gas stream after it is heated and steam is added. U.S. Pat. No. 5,562,754 (Kang et al.) states that the permeate side of the ion transport membrane may be swept with steam. A stream of oxygen-containing gas is heated in a direct-fired combustor, passed through the retentate zone of the ion transport membrane, and then directed to a gas turbine to generate power. This non-permeate stream is then discarded as exhaust.